Condensation soldering is a process wherein the vapor of a high temperature boiling point liquid is allowed to contact the objects or components to be soldered. The latent heat of evaporation given up when the vapor condenses on the object quickly raises its temperature and causes any solder, tin, alloy or tin-lead electroplate or other similar material thereon to melt and flow. It is particularly desirable to accomplish this end with minimal loss of the relatively expensive heat transfer vapor from the facility. In addition, escape of other vapors, such as those produced during the soldering process, should be kept to a minimum from an environmental standpoint.
Although the background of the present invention is most readily understood in the context of soldering, its application is not to be construed as limited to soldering. Many processes, such as curing, cooking, fusing and brazing, as well as soldering, require that articles be rapidly heated to elevated temperatures. Additionally, other types of processes besides heating processes also require that a vapor be contained within a tank or vessel as a conveyor moves therethrough (such as defluxers, degreasers or the like).
Several methods and apparatus have been disclosed in the prior art for effecting solder reflow operations on printed circuits through the use of hot saturated vapors. One such facility is disclosed in R. C. Pfahl, Jr. et al., U.S. Pat. No. 3,866,307, issued Feb. 18, 1975 and is incorporated by reference herein. Circuit boards are loaded onto a conveyor and moved downward into a chamber containing hot saturated vapor of a high boiling point heat transfer fluid such as a fluorinated hydrocarbon. As the circuit boards pass through the vapor they are heated to a suitable temperature for soldering. The circuit boards then travel upward and out of the chamber where the solder cools and solidifies to form a bond. Such a technique has proved most effective for soldering, fusing and the brazing of articles.
Additional "in-line" type systems have been developed such as those shown in U.S. Pat. Nos. 4,321,031 to R. W. Woodgate and 4,389,797 to D. J. Spigarelli et al. Each patent describes an in-line condensation soldering system having a centrally located heating chamber containing hot saturated vapor. Both systems have an input and exit throat which laterally communicate with the heating chamber. In operation, one or more conveyors carry articles to be soldered through the input throat, into the heating chamber to reflow the solder on the articles and along the exit throat where the articles may be offloaded.
An additional commercially available system also has a heating chamber with input and exit throats but the heating zone is in the lower portion of the heating chamber. This requires the conveyor to bring the articles to be soldered down, into the vapor heating zone to reflow the solder, then up and into the exit throat. All of the described systems have cooling surfaces proximate to the input and output throats to condense heat transfer vapor to define the heating zone. The condensed vapor can then be directed back to a sump for revaporization within the heating chamber.
Air is drawn from the throat ends to provide ventilation of these areas for operator safety. Vapor escaping from the open throat ends tends to be entrained in this flow and will be prevented from entering the shop environment. In some facilities the flow is processed to remove as much of the heat transfer fluid component therefrom as possible before being passed to exhaust.
One technique used in the above-described apparatus for lessening loss of the expensive vapor is to minimize the disturbance of air/vapor in the interior portions of the faciliites by limiting air motion to the extreme ends of the throats or external areas close to the throat ends. Thus, disturbance of the vapor zone within the facility is kept to a minimum, which lessens the tendency for vapor to move to the throat ends and be lost to the atmosphere, either through the exhaust system or to the shop environment.
This approach has been found to be ineffective since convection of vapor to the open throat areas, in particular at the exit end, is increased by the motion of the conveyor and the parts thereon. Diffusion of the vapor, although very slow, will occur even without convective motion. With air movement limited to the extreme ends of the throats, the flow is difficult to control or contain. Thus, loss of vapor, particularly that evaporating from the surface of the article, can occur to the outside atmosphere, even when the system is designed to draw outside air for ventilation. Recovery of vapor entering the ventilation system is extremely difficult due to the relatively large flow rate and small concentration of vapor.